1. Field of the Invention
The invention relates to a variable valve apparatus for controlling a variable valve apparatus such as a VVT (variable valve timing system), or the like.
2. Description of the Related Art
As for the foregoing type of variable valve apparatus, Japanese Patent Application Publication No. 5-156972 (JP-A-5-156972) discloses a mechanism that is capable of continuously changing the phase of rotation, that is, the displacement angle, of each of an intake-side camshaft and an exhaust-side camshaft via a working fluid, such as oil or the like, relative to the rotation phase of a crankshaft. More concretely, the variable valve apparatus as described above, for example, has a rotor which has vanes that are provided along an outer peripheral portion of the rotor, and which is fixed to an intake-side camshaft, and a housing which houses the rotor, and to which rotation force is transmitted from the crankshaft. In this apparatus, the vanes are fixed to the housing or moved relative to the housing in the rotation direction, according to the hydraulic pressure of a working fluid, such as oil or the like, that is supplied into the housing. When the internal combustion engine is stopped, a lock pin fits into a lock hole that is formed in one of the vanes, which are fixed at a predetermined position, so that the rotation phase of the intake-side camshaft and the rotation phase of the crankshaft are fixed to each other. In conjunction with the variable valve apparatus having a mechanism as described above, a technology capable of reducing the decrease in the effective compression ratio while reducing the pumping loss at low temperature, by advancing the opening/closing timing of the intake valves.
Besides, Japanese Patent Application Publication No. 2003-328789 (JP-A-2003-328789), Japanese Patent Application Publication No. 2007-32415 (JP-A-2007-32415) and Japanese Patent Application Publication No. 2004-169646 (JP-A-2004-169646) disclose a technology in which if cooling water that cools the internal combustion engine has high temperature, the combustion of fuel in the combustion chambers is stabilized by setting the displacement angle of the exhaust valves to the most retarded angle or changing the amount of the overlap between the open period of the intake valves and the open period of the exhaust valves.
However, according to the variable valve apparatus that has a mechanism capable of continuously changing the rotation phase of the intake-side camshaft and the rotation phase of the exhaust-side camshaft via a working fluid such as oil or the like, for example, when the temperature of the oil is low, it becomes difficult to fix the rotation phases of the crankshaft and the intake-side camshaft at the time of stop of the internal combustion engine, since the viscosity of oil increases as the temperature of oil becomes lower. More concretely, for example, in the case where the viscosity of oil as a working fluid is increased, there occurs a relative decline in the speed at which the vanes of the rotor move within the oil in the rotation direction of the intake-side camshaft according to friction when the engine is stopped. If the internal combustion engine stops operating while the vanes are moving at such a reduced speed, the vanes cannot quickly move to the position where the vanes are designed to be fixed when the engine is stopped, and therefore it becomes difficult to accurately fit the lock pin into the lock hole formed in a vane.
In addition, a technology capable of achieving both improved combustion characteristic of an internal combustion engine and reduced production of hydrocarbon (HC) gas by setting the amount of overlap between the open period during which the intake valves are open and the open period during which the exhaust valves are open to an appropriate value is also desired.